Amplifiers are used in a variety of applications, including devices that transmit and receive wireless signals. One particular type of power amplifier often used in radio frequency (RF) communication transceiver circuits is the Doherty amplifier.
The Doherty amplifier power-combines two separate amplifiers: a carrier amplifier and a peaking amplifier, each of which is biased differently. The carrier amplifier operates as a normal class AB amplifier, while the peaking amplifier operates as class C amplifier. An RF input signal is split (e.g., using a quadrature coupler), and is sent to both the carrier amplifier and the peaking amplifier, which separately operate to amplify their respective inputs. The outputs of the carrier amplifier and the peaking amplifier are then combined to provide a final amplified signal. In operation, the carrier amplifier operates most of the time, handling average input signals, while the peaking amplifier operates only when peak power is needed.
The design of amplifiers (including power amplifiers) typically involves trade-offs in various device parameters. In general, device technology process optimization involves trade-off of competing parameters such as power, density, gain, efficiency, and linearity. Often, improving the power density is a direct trade-off for linearity. In an amplifier, power density is important for minimizing the drain-source capacitance (Cds) and the gate-source capacitance (Cgs) per watt of the amplifier. This allows for improved bandwidth, and also allows more power to be used in a given circuit footprint, which, in turn, can lead to cost reduction in constructing the amplifier.
Linear amplifiers that are used in amplifier architectures such as a Doherty amplifier generally require good linearity in their operation. This is because in a Doherty amplifier, the carrier amplifier must operate the majority of the time of a load state at close to maximum efficiency, which corresponds to the worst-case linearity for the amplifier. Furthermore, in Doherty amplifiers the peaking amplifier must operate in class C or low class B, which also degrades linearity.
Conventional amplifiers use a single, constant gate and drain voltage fed to an entire die and/or die fingers in a given amplifier path. However, different circuit elements of different positions on the die do not operate identically, this can lead to a loss in linearity. For example, RF current density will be higher at the outer edges of package leads, meaning that center paths are not equivalent to edge die pads. This is also true from a thermal standpoint where temperature gradients of 40° C. or higher are possible.
It would therefore be desirable to provide a way to improve the linearity of RF amplifier circuits, while minimizing the associated loss of power density.